1. Field of the Invention
This invention relates generally to the field of mass spectrometry and, more specifically, to the reduction of cross-talk between RF components of a mass spectrometer.
2. Description of the Related Art
Nowadays, RF components are standard devices for use in mass spectrometry. Examples of RF components used in a mass spectrometer include multipole ion guides, multipole mass analyzers (sometimes also called mass filters), pre/post filters, multipole collision cells, and multipole ion traps. Such RF components may be implemented using a configuration having an even number of elongate pole electrodes arranged equi-angularly on a circular perimeter about a common axis. This axis may be linear or non-linear, such as curved. Some mass spectrometers use RF components in tandem or adjacent to one another. Examples of such tandem devices can be found in U.S. Pat. No. 6,191,417 B1 (Douglas et al.) and U.S. Pat. No. 6,340,814 B1 (John Vandermey) where a tandem quadrupole mass filter assembly is disclosed. U.S. Pat. No. 6,576,897 B1 (Steiner et al.) shows a triple quadrupole mass analyzer with a curved ion collision cell which is operated in a so-called RF only mode.
The close proximity of the RF components results in RF coupling or cross-talk therebetween, which causes unwanted perturbations from one RF component on the other adjacent RF component. As a result of these external perturbations, the system performance of the mass spectrometer is degraded. For example, external perturbations on a mass analyzer as a consequence of RF coupling with an adjacent RF component can cause the mass resolution of the mass analyzer to change. Because resolution is related to the ion transmission of the mass analyzer, the overall sensitivity of the measurement will also be affected, which is undesirable.
One approach of overcoming the issues associated with cross-talk between adjacent RF components is placing one or more electrostatic lenses between them. A lens usually consists of a conductive sheet with an aperture and provides a shielding or screening effect impeding the RF voltages of one RF component cross-talking to the other RF component and vice versa. However, due to the lenses being arranged in between the end-faces of the adjacent RF components they also influence the ion transmission characteristics by, for instance, reducing the geometrical acceptance of the respective downstream RF component and also by creating an additional surface where stray ions can hit, charge-up and create an electric field distortion. The latter, in particular, increases the optimization complexity of the instrument.
Another approach of overcoming cross-talk or capacitive coupling is described in U.S. Pat. No. 8,314,385 B2 (Roy Moeller). Some of the electrodes of one RF component are provided with axial extensions which in part spatially overlap with angularly offset electrodes of the other RF component, however, without establishing electrical contact therewith. The overlap area and distance between extensions and electrodes is chosen such as to compensate for, preferably any, capacitive coupling between the adjacent RF components. This design generally works well, but requires additional effort and expense when fabricating the multipole electrodes to also include the extensions, and properly align them with those of another multipole RF component.
Hence, there is still a need for technically simple and economic means to reduce cross-talk or capacitive coupling between adjacent RF components in a mass spectrometer, however, without suffering the negative effects of geometrical acceptance degradation and/or (too much) electric field distortion.